The invention relates to spray powder-coating apparatus defined in the preamble of claim 1.
Such a spray powder-coating apparatus is known from the European patent document 0 686 430 A.
The European patent document 0 636 420 discloses spray powder-coating apparatus fitted with an electronic regulating system generating setpoint signals for the required rate of powder, i.e. the quantity of powder per unit time, as a function of a setpoint value, and for the rate of total air to be conveyed, i.e. the quantity of total air per unit time, which is required to move the powder, said setpoints being applied to pressure regulators which then correspondingly regulate the feed of conveyance air and of supplemental air to an injector. The setpoint signals from the regulation system are construed as setpoint values by the regulators and are utilized in relation to the actual values of the conveyance air or of the supplemental air to regulate said conveyance or supplemental air. Volumetric regulators may also be used instead of the pressure regulators.
A pneumatic powder conveyance system is known from U.S. Pat. No. 4,747,731 (corresponding to the European patent documents 0 239 331 A and 0 423 850 A), which comprises 2 injectors of which the main injector is mounted at the downstream end and an auxiliary injector is mounted at the upstream end of a powder aspirating tube.
It is known from U.S. Pat. No. 5,186,388 to measure the partial vacuum in the partial-vacuum zone of an injector and to use this measurement as being the powder rate. It is known from U.S. Pat. No. 4,544,306 to use a measuring tube having one end open to the atmosphere and another open end opening into a powder/air duct to measure the pressure therein. Depending on the pressure relative to atmospheric generated by the powder/air flow, a valve shall be opened or closed at the powder feeding outlet situated at the lower funnel-shaped end of a powder supply cart.
Air dividers are known from U.S. Pat. No. 3,625,404 and from German patent document 44 09 493 A which contain a throttling valve in a conveyance air line and a throttling valve in a supplemental air line, said valves being mechanically coupled to each other. To the extent one of said valves shall close the other one shall open.
The objective of the present invention is to create accurate and stable regulation of the pneumatically conveyed flow of powder as a function of a manually or automatically preset setpoint value for the rate of applied powder, without thereby requiring expensive pressure regulators or volumetric controls.
This problem is solved by the invention by means of the features of claim 1.
The invention offers economical apparatus of simple design which enables automated and accurate regulation of a powder/air flow and allowing stable air flow of powder/air, free of pulsations, from start to shutdown.
The concepts of values such as xe2x80x9creference value, actual value, and/or setpoint value . . .xe2x80x9d used in the present disclosure shall denote, depending on the desired design of the apparatus, the value at a point or of a range of values. However even as regards a value at a point, the tolerance-entailed fluctuations in value still shall be within the scope of the invention.